The invention concerns a hydraulic cylinder that locks into position, with a housing with at least one pressure connection and a piston and rod that travel tight in an out of the housing, whereby the locking mechanism comprises a piston that rests against a spring and a barrier mechanism with a cage of balls that roll radially into a locking position in a locking groove and back into an unlocking position in a locking groove, and whereby the primary piston communicates with a hollow space and acts on a primary participating surface and the locking piston communicates with an unlocking space and acts on an unlocking surface by way of the pressure connection. Hydraulic lock-into-position cylinders are employed to activate structures--a lid, a lever, or a similar component for example--that are articulated to the rod of the primary piston and can be activated by applying force to the piston. The piston can be forced in one or both directions and can even be operated differentially. The present invention, however, is unaffected by which of these modes are employed. It is necessary to be able to mechanically lock the primary piston into a ready position when it is not being subjected to pressure through the pressure connection. It must also be possible to reliably attain the locking potion once the primary piston has traveled back into its ready position due to pressure against the rod end if the cylinder is double-action or to the weight of the output component on the rod if the cylinder is single-action.
Hydraulic lock-into-position cylinders of the aforesaid type are known from German Patent 2 911 071 and German OS 3 732 561. The cylinder disclosed in the patent has a primary piston that travels in and out of the housing and has a locking surface that communicates with a hollow space and can be directly subjected to pressure at the housing by way of the pressure connection. The primary piston extends into a cage for the balls in a barrier mechanism that is surrounded by a sleeve-like locking piston. The return-space end of the piston rests against a locking spring. Since the locking piston has an unlocking surface that can also be subjected in communication with an unlocking space by way of a line that branches off the pressure connection, pressure can build up in the primary space and in the unlocking space simultaneously. The primary piston, however, cannot as yet leave its ready position because the barrier mechanism is still in the locking position. Not until the locking piston has traveled out of the locking position and into the unlocking position against the force of the locking spring can the balls in the barrier mechanism move radially out of the locking groove and into the unlocking groove, and only then can the primary piston travel. The direct subjection of the locking surface of the primary piston to pressure from the pressure connection, however, subjects the balls in the barrier mechanism to force in the locking position and squeezes them to a certain extent because the primary piston transmits tension through the balls even though it cannot leave its ready position. The sudden subjection of the overall unlocking surface of the locking piston to pressure accelerates the locking piston into the unlocking position, and the balls in the barrier mechanism leap out radially just as suddenly, with the result that the primary piston begins to move suddenly. This is in many cases undesirable and even a drawback to the output component. Another drawback is that air cannot leave the reversing space in the locking piston, which accommodates the locking spring, and the barrier mechanism may become hydraulically blocked if the seal on the locking piston is loose and allows fluid to enter and occupy the reversing space. In this case as well the reversing space in the locking piston, which accommodates the locking spring, constitutes a closed volume. If the balls are distributed inside the barrier mechanism such that they can be accommodated as in a cage in a sleeve that projects out of the primary piston, there is another drawback in that they can rub against the cylindrical wall of the housing while the primary piston is traveling, and the resulting damage will lead to leakage on the part of the seal on the primary cylinder. If, on the other hand, the balls in the barrier mechanism are mounted stationary on an extension and do not participate in the axial motion of the primary piston, this drawback will be absent, although the primary space and the unlocking space will still be simultaneously subjected to hydraulic pressure, and the primary piston will again begin to move suddenly once it has been subjected to pressure and the barrier mechanism has disengaged.